1. Field of the Invention
The present invention relates to an EL panel in which an EL element formed on a substrate is sealed between the substrate and a cover member, and to a method of driving the EL panel. The invention also relates to an EL module obtained by mounting an IC to the EL panel, and to a method of driving the EL module. The EL panel and the EL module are generically called light emitting devices in this specification. Also, in the present invention electronic machines using light emitting devices that display images when driven by the driving methods are included.
2. Description of the Related Art
Being self-luminous, EL elements eliminate the need for a backlight that is necessary in liquid crystal displays (LCDs) and thus make it easy to manufacture thinner displays. Also, the self-luminous EL elements are high in visibility and have no limit in terms of viewing angle. These are the reasons for attention that light emitting devices using the EL elements are receiving in recent years as display devices to replace CRTs and LCDs.
An EL element has a layer containing an organic compound that provides luminescence (electroluminescence) when an electric field is applied (the layer is hereinafter referred to as EL layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds is classified into light emission upon return to a base state from singlet excitation (fluorescence) and light emission upon return to a base state from triplet excitation (phosphorescence). A light emitting device according to the present invention can use both types of light emission.
All the layers that are provided between an anode and a cathode are an EL layer in this specification. Specifically, the EL layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc. A basic structure of an EL element is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order.
In this specification, an EL element emitting light is expressed as an EL element being driven. The EL element as defined herein is a light emitting element that is composed of an anode, an EL layer, and a cathode.
Methods of driving a light emitting device having an EL element are roughly divided into analog driving methods and digital driving methods. Digital driving is deemed more promising in view of transition from analog broadcasting to digital broadcasting since it enables the light emitting device to display an image using a digital video signal that carries image information as it is without converting the signal into an analog signal.
There are two types of gray scale display methods that utilize binary voltages of digital video signals: one is an area ratio driving method and the other is a time division driving method.
The area ratio driving method is a driving method in which a pixel is divided into a plurality of sub-pixels and each sub-pixel is individually driven in accordance with a digital video signal to obtain gray scale display. Since the area ratio driving method involves dividing one pixel into plural sub-pixels and driving each sub-pixel individually, a pixel electrode is needed for every sub-pixel. This complicates the pixel structure, causing inconveniences.
The time division driving method, on the other hand, is a driving method that provides gray scale display by controlling the length of time pixels are lit. Specifically, one frame period is divided into a plurality of sub-frame periods. In each sub-frame period, to be lit or not is determined for the respective pixels in accordance with digital video signals. The accumulated lengths of sub-frame periods during which a pixel is lit with respect to the length of the entire sub-frame periods in one frame period determine the gray scale of that pixel.
Organic EL materials in general have faster response speed than liquid crystals, which makes an EL element suitable for time division driving.
Described below is the pixel structure of a common light emitting device driven by time division driving. The description is given with reference to FIG. 25.
FIG. 25 is a circuit diagram of a pixel 9004 of a common light emitting device. The pixel 9004 has one of source signal lines (source signal line 9005), one of power supply lines (power supply line 9006), and one of gate signal lines (gate signal line 9007). The pixel 9004 also has a switching TFT 9008 and an EL driving TFT 9009. The switching TFT 9008 has a gate electrode connected to the gate signal line 9007. The switching TFT 9008 has a source region and a drain region one of which is connected to the source signal line 9005 and the other of which is connected to a gate electrode of the EL driving TFT 9009 and to a capacitor 9010. Each pixel of the light emitting device has one capacitor.
The capacitor 9010 is provided to hold the gate voltage (the difference in electric potential between the gate electrode and a source region) of the EL driving TFT 9009 when the switching TFT 9008 is not selected (when the TFT 9008 is in an OFF state).
The source region of the EL driving TFT 9009 is connected to the power supply line 9006 whereas a drain region thereof is connected to an EL element 9011. The power supply line 9006 is connected to the capacitor 9010.
The EL element 9011 comprises of an anode, a cathode, and an EL layer placed between the anode and the cathode. If the anode is in contact with the drain region of the EL driving TFT 9009, the anode serves as a pixel electrode whereas the cathode serves as an opposite electrode. On the other hand, the cathode serves as the pixel electrode whereas the anode serves as the opposite electrode if the cathode is in contact with the drain region of the EL driving TFT 9009.
The opposite electrode of the EL element 9011 is given with an opposite electric potential. The power supply line 9006 is given with a power supply electric potential. The power supply electric potential and the opposite electric potential are provided by a power source placed in an external IC to the display device.
The operation of the pixel shown in FIG. 25 is described next.
A selection signal is inputted to the gate signal line 9007 to turn ON the switching TFT 9008, through which a digital signal carrying image information (hereinafter the signal is referred to as digital video signal) and inputted to the source signal line 9005 is inputted to the gate electrode of the EL driving TFT 9009.
The digital video signal inputted to the gate electrode of the EL driving TFT 9009 contains information, which is ‘1’ or ‘0’ and used to control switching of the EL driving TFT 9009.
When the EL driving TFT 9009 is turned OFF, the electric potential of the power supply line 9006 is not given to the pixel electrode of the EL element 9011 and therefore the EL element 9011 does not emit light. On the other hand, when the EL driving TFT 9009 is turned ON, the electric potential of the power supply line 9006 is given to the pixel electrode of the EL element 9011 to cause the EL element 9011 to emit light.
The above operation is conducted in each pixel, whereby an image is displayed.
In the light emitting device that displays an image through the above operation, however, the luminance of the EL element changes when the temperature is changed in the EL layer of the EL element due to the temperature of the surroundings or heat generated from the EL panel itself. FIG. 26 shows a change in voltage-current characteristic of the EL element when the temperature of the EL layer is changed. The current flowing through the EL element is reduced as the temperature of the EL layer is lowered. On the other hand, the current flowing through the EL element is increased as the temperature of the EL layer is raised.
The less the current flows in the EL element, the more the EL element loses the luminance. The more the current flows in the EL element, the more the EL element gains the luminance. Accordingly, the luminance of the EL element is changed when a change in temperature causes a shift in amount of current flowing in the EL layer even though the voltage applied to the EL element is constant.
The degree of change in luminance due to temperature change varies between EL materials. Therefore, if different EL materials are used in different EL elements in order to emit light of different colors for color display, a change in temperature can cause varying degree of changes in luminance in the EL elements of different colors to make it impossible to obtain desired color.